A wide variety of instruments exist that display a measured parameter by displaying a voltage that is proportional to the measured parameter. For example, in the field of patient monitoring, sensors are often used that provide a voltage that is indicative of a physiological parameter that is being measured. The sensor is connected to a monitor that provides a digital readout of the sensor voltage, and hence the value of the measured physiological parameter. The voltage output by the sensor is essentially a DC voltage that changes in accordance with changes of the measured physiological parameter. However, for a variety of reasons, the DC voltage often contains a small AC component, known as ripple. While the ripple does not normally degrade the accuracy of the measurement to any significant degree, the ripple does make it difficult to read the measured parameter because the digital display responds to the ripple. Thus, for example, for a measured heart rate of 75, the display might jump rapidly back and forth through the three heart rate values 74, 75 and 76. This rapid changing of the display--known as "display jitter"--makes it difficult to read any of the three displayed values.
In the past, attempts have been made to prevent the monitor from responding to ripple by filtering the ripple using a low-pass filter. While a low-pass filter can attenuate ripple sufficiently to avoid display jitter, it often creates an additional problem for which there has been no satisfactory solution. Specifically, passing the voltage from a sensor through a low-pass filter limits the slew rate, i.e., the rate at which the filter can respond to changes in the input signal. Thus, while a low-pass filter having a sufficiently low cutoff frequency can eliminate ripple, it also limits the ability of the filter to respond to rapid and substantial changes in the input voltage. As a result, the monitor does not respond to changes in the physiological parameter as rapidly as they occur and are measured by the sensor. Instead, a sudden change in the measured physiological parameter is perceived on the monitor as a slow change in the physiological parameter. Thus, during these rapid and/or substantial changes in the physical parameter, the monitor does not accurately display the value of the measured physiological parameter.
In the past, there has been no satisfactory solution to the problem of eliminating display jitter induced by ripple and still allowing a monitor to accurately respond to the true value of a physiological parameter. Instead, it was always necessary to trade off display jitter with display response time.